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Jeep Service, Jeep Owner

Category: Jeep

Satisfied Customers: 4608

Experience: 14 years with AMC/Jeep Specializing with pre-1994 Jeep products.

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1976 Jeep CJ7 4.2L I6. Newly machined head with valve job,

Customer Question

1976 Jeep CJ7 4.2L I6. Newly machined head with valve job, weber 2bbl carb, new fuel filter and fresh ethanol free fuel, prestolite distributor with vacuum advance, initial timing set at 8*, vacuum and mechanical advance seems to be working, it fires right up and idles fairly well, but any acceleration at all results in popping through the carb. It does this hot or cold and the more gas you give it, the louder the pop. The popping does seem to be rythmic not random. Adjusting timing does not seem to help.

The popping through the carb happens the same way, with or without the vacuum advance attached, at the slightest acceleration. "Road testing" consists of idling to the corner and back as the popping sounds like it could be doing a great deal of damage.

If you are getting popping, your cam timing is incorrect. Set the engine to TDC. Pull the cap off and examine where the end of the distributor rotor is. Advise exactly where it is in relation to the #1 cylinder tower inside the distributor cap.

Correct. the crank-to-cam timing via the timing chain is absolutely critical. When the mechanical advance moves the distributor base plate, the spark happens at a different instant in time. What you have, on acceleration, is the spark occurring before the piston reaches TDC on the firing stroke. The intake valves are being allowed to move to the closed position as the piston starts upward. If the cam timing is off 1 tooth, the vehicle will start. As the spark occurs (in this case, at the right time via distributor placement, but, at the wrong time with regard to valve placement), the mixture fires in the cylinder. If the intake valves are not fully closed, the compression of the mixture igniting takes the path of least resistance. In your case, it is blowing past the slightly open valve into the intake manifold. This is the popping. The recommendation I have for you is to pull the timing cover and recheck your cam timing.

Burnt or otherwise damaged intake valve. Bad intake valve seat. Rockers & pivots in the head incorrect. Incorrect push rods. Anything that allows either compression to slide past the valve (if it is fully closed) or something that is not allowing the valve to close completely. Popping back thru the intake is not something that we could call common on the 4.2/258s.

Get someone to help you. Have them hold the idle fast enough to make it pop. Using insulated pliers, pull off the spark plug boots one at a time. When the popping quits, you have found the cylinder that is the problem. If it pops on all of the cylinders, the cam has to be off a tooth.

I have a compression gauge that I am borrowing in the morning and will check compression tomorrow. I will also try isolating the cylinder via the plug wires. Pulling the timing cover is not something i'm looking forward to. Also, the head just came back from the machine shop, cleaned, magnafluxed (no cracks), decked, valve job, new seals. Three new push rods (same length as the others) to replace slightly bent ones, and new rockers to replace slightly worn ones. Unfortunately, I did not think to pull the lifters as I believed it to only be a blown head gasket.